This invention relates to the combination of a thick film, inorganic, electroluminescent (EL) lamp and a power source and, in particular, to the construction of an EL lamp for improved brightness, efficiency, and life.
In the prior art, brightness and efficiency are inversely related, as are brightness and life. Unfortunately, the market demands that all three increase simultaneously or at least that brightness increase without decrease in efficiency or life.
An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer can include a phosphor powder or there can be a separate layer of phosphor powder adjacent the dielectric layer. The phosphor powder radiates light in the presence of a strong electric field, using relatively little current.
A modern (post-1990) EL lamp typically includes transparent substrate of polyester or polycarbonate material having a thickness of about 7.0 mils (0.178 mm.). A transparent, front electrode of indium tin oxide or indium oxide is vacuum deposited onto the substrate to a thickness of 1000 Axc2x0 or so. A phosphor layer is screen printed over the front electrode and a dielectric layer is screen printed over phosphor layer. A rear electrode is screen printed over the dielectric layer. It is also known in the art to deposit the layers by roll coating; e.g. see U.S. Pat. No. 5,830,028 (Zovko et al.).
The inks used for screen printing or roll coating include a binder, a solvent, and a filler, wherein the filler determines the nature of the printed, layer. A typical solvent is dimethylacetamide (DMAC) or ethylbutylacetate (EB acetate). The binder is typically a fluoropolymer such as polyvinylidene fluoride/hexafluoropropylene (PVDF/HFP), polyester, vinyl, epoxy or Kynar 9301, a proprietary terpolymer sold by Atofina. A phosphor layer is typically screen printed from a slurry containing a solvent, a binder, and zinc sulphide particles. A dielectric layer is typically screen printed from a slurry containing a solvent, a binder, and barium titanate (BaTiO3) particles. A rear (opaque) electrode is typically screen printed from a slurry containing a solvent, a binder, and conductive particles such as silver or carbon. Because the solvent and binder for each layer are chemically the same or similar, there is chemical compatibilityand good adhesion between adjoining layers.
Because an EL lamp is a capacitor, alternating current must be applied to the electrodes to cause the phosphor to glow, otherwise the lamp charges to the applied voltage, the current through the EL lamp ceases, and the lamp stops producing light. In portable electronic devices, automotive displays, and other applications where the power source is a low voltage battery, an EL lamp is powered by an inverter that converts low voltage, direct current into high voltage, alternating current. In order for an EL lamp to glow sufficiently, a peak-to-peak voltage in excess of about one hundred and twenty volts is necessary. The actual voltage depends on the construction of the lamp and, in particular, the field strength within the phosphor powder. The frequency of the alternating current through an EL lamp affects the life of the lamp, with frequencies between 200 hertz and 1000 hertz being preferred. Ionic migration occurs in the phosphor at frequencies below 200 hertz, leading to premature failure. Above 1000 hertz, the life of the phosphor is inversely proportional to frequency.
The prior art discloses several types of inverters in which the energy stored in an inductor is supplied to an EL lamp as a small current at high voltage as the inductor is discharged either through the lamp or into a storage capacitor. The voltage on a storage capacitor is pumped up by a series of high frequency pulses from the inverter. The direct current produced by inverter must be converted into an alternating current in order to power an EL lamp. U.S. Pat. No. 4,527,096 (Kindimann) discloses a switching bridge, known as an xe2x80x9cH-bridge,xe2x80x9d for this purpose. The H-bridge acts as a double pole double throw switch to alternate current through the EL lamp at low frequency. U.S. Pat. No. 5,313,141 (Kimball) discloses an inverter that produces AC voltage directly. A plurality of inverters are commercially available using either technology.
Curiously, a bench power supply, i.e. a power supply coupled to line voltage, and an inverter are not equivalent power sources for operating an EL lamp. The brightness of some EL lamps may be greater on one supply or the other. In general, bench supplies have a generally constant, sinusoidal output voltage and provide whatever current is necessary to power an EL lamp. An EL lamp for bench supplies is designed for higher current to increase brightness. Inverters generally have limited current capability, variable output voltage, and high harmonic distortion, i.e. the output waveform is not sinusoidal.
In any circuit with impedances, such as an EL lamp, the current and the voltage are usually not in phase. Current is taken as a reference and the voltage either leads or lags the current. Mathematically, such a circuit is represented with complex numbers (axc2x1jb) having a real or resistive component, a, and an imaginary or reactive component, b. Calculating true power can be difficult and the term xe2x80x9capparent powerxe2x80x9d is used for the product of the rms (root mean square) values of the real components of voltage and current.
In view of the foregoing, it is therefore an object of the invention to improve the brightness of an EL lamp without decreasing efficiency.
Another object of the invention is to improve the brightness of an EL lamp without decreasing lamp life.
A further object of the invention is to increase brightness and efficiency of an EL lamp driven from a current limited source.
Another object of the invention is to improve the efficiency of an EL lamp in terms of brightness per milliwatt of apparent power per unit area.
The foregoing objects are achieved in this invention in which it has been discovered that brightness is improved, without decreasing lamp life, by driving an EL lamp at higher voltage and less current, by increasing phosphor loading, and by printing the phosphor layer in two or more passes to produce a phosphor density of at least 6.5 milligrams per square centimeter of lamp surface area. A single, thicker layer can be deposited by roll coating.